The present invention relates to communication systems, and more particularly to methods and apparatuses for estimating the peak value of a sampled signal in a communications system.
In a wireless digital telecommunication system, such as a Wideband Code Division Multiple Access (W-CDMA) system, a discrete signal is sent from a transmitter to a receiver via a channel. The signals in the transmitter and the receiver are processed in a digital format, but they are in an analog format when they traverse the channel. Consequently, a typical transmitter/receiver arrangement typically has the form illustrated in FIG. 1. In the transmitter 101, the digital signal to be transmitted is supplied to a Digital-to-Analog (D/A) converter 103, which generates an analog form of the signal. The analog signal is then processed by a bandpass filter 105 to eliminate frequency components that are outside of the desired frequency spectrum to be used during the transmission. The impulse response of the bandpass filter 105 is represented herein by the function g1(t). The filtered signal is then supplied to the channel 107 (e.g., by means of an antennaxe2x80x94not shown).
The channel 107 has its own characteristics which further modify the signal. These characteristics may be modeled by a filter 109, having an impulse response h(t). In addition to the channel""s inherent characteristics, noise is usually present in the channel which also combines with the signal, as represented by the adder 111.
The channel 107 then supplies the resultant signal to the receiver 113 (e.g., by means of an antennaxe2x80x94not shown). The receiver 113 processes the received analog signal through a bandpass filter 115 to remove unwanted frequency components. The receiver""s bandpass filter 115 has an impulse response that is represented herein by the function g2(t). The filtered received signal may then be supplied to an Analog-to-Digital converter (A/D converter) 117, which converts the signal back into a digital form (e.g., by sampling the amplitude of the filtered received signal at regular intervals of time).
FIG. 2 is a graph that depicts the relationship between an exemplary analog signal (having a continuous set of values over a period of time) and an exemplary set of sampled values, yx, determined at corresponding sampling times x. The time interval, T, between consecutive sampling times is typically constant for all sampling times.
It is often important to determine the peak value(s), yp, of the analog signal 201. For example, in telecommunication systems, the peak values may be used to correctly estimate signal-to-noise ratios (S/N) or, for spread spectrum systems, signal-to-interference-and-noise ratios (SINR). However, as may be seen by the example of FIG. 2, the sampled values, yx, may not include a value that was determined exactly at the moment, sp, that the analog signal 201 exhibited a peak value yp. At best, the sampled values, yx, may only include two values (e.g., the values y1 and y2) that were determined at respective times s1 and s2 that are just on either side of the peak, yp.
It is possible to estimate the peak, yp, simply by using the largest sample value (e.g., the value y1 shown in FIG. 2) without modification. However, this approach yields results that are either more or less accurate, depending on how far the largest sample value is from the actual peak value. In general, the peak value will be underestimated. This can result in degraded performance in systems that rely on the estimated peak values, such as telecommunications that use it for estimating a SIR.
Consequently, it is desired to have techniques that more accurately estimate the peak value of an analog signal from a set of sampled values.
It should be emphasized that the terms xe2x80x9ccomprisesxe2x80x9d and xe2x80x9ccomprisingxe2x80x9d, when used in this specification, are taken to specify the presence of stated features, integers, steps or components; but the use of these terms does not preclude the presence or addition of one or more other features, integers, steps, components or groups thereof.
In accordance with one aspect of the present invention, the foregoing and other objects are achieved in methods and apparatuses for estimating a peak magnitude of a signal from a set of signal samples that have been communicated through a communication system. Making the estimation includes selecting, from the set of signal samples, a largest signal sample that occurs at a time, s1; and selecting, from the set of signal samples, a next largest signal sample that is adjacent to the largest signal sample, wherein the next largest signal sample occurs at a time, S2. A first ratio of a first derived signal sample and a second derived signal sample is then generated, wherein the first derived signal sample is derived from the largest signal sample, and the second derived signal sample is derived from the next largest signal sample. The first ratio is used to determine a second ratio, wherein the second ratio represents a peak magnitude of a communication system response divided by a second magnitude of the communication system response, wherein the second magnitude of the communication system response is the communication system response at the time s1. The second ratio and the first derived signal sample are used to generate the peak magnitude of the signal.
In some embodiments, selecting, from the set of signal samples, the largest signal sample and the next largest signal sample that is adjacent to the largest signal sample are performed based on magnitude of the set of signal samples without regard to sign.
In some embodiments, the first derived signal sample is equal to the largest signal sample; and the second derived signal sample is equal to the next largest signal sample that is adjacent to the largest signal sample.
Using the second ratio and the first derived signal sample to generate the peak magnitude of the signal may include multiplying the second ratio by the first derived signal.
In some alternative embodiments, the first derived signal sample is generated by subtracting a noise estimate from the largest signal sample; and the second derived signal sample is generated by subtracting the noise estimate from the next largest signal sample that is adjacent to the largest signal. In such embodiments, generating the noise estimate may be performed by determining a mean value of a remaining set of signal samples from the set of signal samples.
Also in such embodiments, using the second ratio and the first derived signal sample to generate the peak magnitude of the signal may include adding the noise estimate to a product of the second ratio and the first derived signal.
In some embodiments, using the first ratio to determine the second ratio may include using the first ratio to locate the second ratio in a lookup table; and retrieving the second ratio from the lookup table.
In such embodiments, using the first ratio to locate the second ratio in the lookup table may include locating a first entry in the lookup table having stored therein the first ratio, and determining a second entry in the lookup table that is associated with the first entry, wherein the second entry in the lookup table is associated with the second ratio. Alternatively, using the first ratio to locate the second ratio in the lookup table may include locating a first entry having stored therein a nearest value that is less than the first ratio; determining a second entry in the table that is associated with the first entry; locating a third entry having stored therein a nearest value that is greater than the first ratio; determining a fourth entry in the table that is associated with the third entry; and using the first ratio, the first entry, the second entry, the third entry and the fourth entry to interpolate a value that represents the second ratio.
In another aspect of the invention, the lookup table may be generated by selecting a number, k, of values of the first ratio; and for each of the k values of the first ratio, directly calculating a corresponding value of the second ratio as a function of the first ratio. Alternatively, the lookup table may be generated by selecting a number, k, of values of candidate sample times, s1:s11, . . . , s1k; for each of the values of the candidate sample times, using the communication system response to calculate a value of the first ratio as a function of said each value of the candidate sample time; and for each of the values of the candidate sample times, using the communication system response and the peak magnitude of the communication system response to calculate a value of the second ratio as a function of said each value of the candidate sample time.